Banca de DEFESA: Ítalo Lopes de Oliveira

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : Ítalo Lopes de Oliveira
DATE: 30/08/2022
TIME: 14:00
LOCAL: Sala de videconferência do IGD
TITLE:

Study of the Nepheline-Kalsilite System in Volcanic Rocks of the Goiás Alkaline Province (GAP), Central Brazil

KEY WORDS:

Potassic and Ultrapotassic Magmatism; Mineral Chemistry; Feldspathoids; Alkaline Mafic Rocks; Nepheline Stoichiometry

PAGES: 273
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The Goiás Alkaline Province comprises a wide variety of alkaline mafic and felsic volcanic rocks. Most of the earlier researches emphasised field mapping, petrography, and bulk rock geochemistry. Detailed mineral chemistry is still lacking, particularly in regard to those from the nepheline-kalsilite system, which have guided the current view about the genesis of the GAP magmas. In view of that, a systematic study is being carried out to better understand the uncommon coexistence of nepheline and kalsilite in some of these volcanic rocks, and their significance for the source and origin of this alkaline magmatism in central Brazil. Sampling was focused on gather volcanic and subvolcanic rocks through the province representative of parental magmas. A careful petrographic study permitted to recognize at least two distinct groups: feldspar-free mafic rocks and feldspar-bearing mafic and felsic rocks. Based on the proposed initial objective, the study was focused on detailing the textural and compositional characteristics of the minerals of interest (nepheline and kalsilite) in the feldspar-free mafic rocks only. The results show that potassic nepheline is the dominant groundmass felsic phase of the feldspar-free volcanic rocks and it has a primary magmatic origin as kalsilite. These volcanic rocks cannot be properly named by strictly following any of the current systems. Hence, it has been proposed a new nomenclature scheme. Furthermore, the variations in the composition of potassic nepheline that occurs in the GAP feldspar-free volcanic rocks expand considerably the current understanding about the nepheline solid solutions in nature. At last, the rare mineral assemblage with nepheline and kalsilite as primary igneous phases suggests that the parental magma(s) as well as the mantle source(s) of the GAP feldspar-free alkaline rocks should be somewhat different from that currently hypothesised (i.e. predominantly potassic rather than ultrapotassic / kamafugite).

BANKING MEMBERS:
Externo à Instituição - REINER NEUMANN
Interna - 1555904 - CATARINA LABOURE BEMFICA TOLEDO
Presidente - 347.092.740-53 - JOSE AFFONSO BROD - Durham
Interna - 1644358 - PAOLA FERREIRA BARBOSA
Externo à Instituição - SERGIO DE CASTRO VALENTE - UFRRJ